The present invention relates to oscillators and more specifically, to self-start circuits for such oscillators.
The popularity of portable computers and hand-held devices has made it desirable for new computer products to operate with low power. To accommodate these low power requirements, computer chips often employ circuits that determine when to enter or exit the low-power state. FIG. 1 shows a typical system power supply 100 with a controller, such as an input/output (I/O) controller 102.
The I/O controller 102 also contains power state control logic 104 and a real-time clock (RTC) 106, as well as other peripheral devices (not shown). The controller 102 powers off (zero voltage) the I/O section of the chip during low-power states while the RTC 106 remains powered by either an auxiliary supply 118 or by a battery 118. The RTC 106 is in a different power "well" than the rest of the chip, which allows the RTC 106 to be powered separately. Also in this separate power well is the power state control logic 104 that controls the power state of the system power supply 100.
In most systems with low-power states, the power supply 100 has a main supply 120 and an auxiliary supply 118. The main supply 120 is enabled when the PwrEn input is active. The PwrEn input is controlled by the LP STATE signal 122 of the power state control logic 104. The controller 102 has internal timers and other inputs (not shown) that determine when to enter or exit the low-power state. VccAUX 118 is always active as long as the supply is plugged into the wall AC power in a desktop system or the main battery is present in a portable system.
The battery 114 provides back up to the auxiliary supply 118 if the supply is unplugged from the wall or the main battery is removed from the portable system. The auxiliary 118 and battery 114 supplies are "OR'ed" together by a diode network 108, 110. Since VccAUX 118 is always higher than the battery voltage while the supply 118 is on, it supplies current to VccRTC 124. If the auxiliary supply 118 fails, the battery 114 supplies the current to VccRTC 124. The resistor 112 in series with the diode 110 is a safety requirement for lithium batteries.
VccRTC 120 supplies power to the RTC 106 and to the power state control 104. Both blocks need to have power in order to keep system time and state during lower-power states when the main supply voltages 120 are shut down.
Resume reset (ResumeRST) 126 is an input that detects if the voltage level on the VccRTC 124 has failed because the battery 114 is dead or absent and VccAUX 118 is off. The RC delay on ResumeRST 126 is used to reset the state of the power state control logic 104, which allows the system to boot up. The ResumeRST signal 126 also enables a START signal to the RTC to ensure that the oscillator 116 is running.
When a battery is replaced or first inserted by a consumer or manufacturer, the oscillator may not self-start and thus needs "kicking." Since the consequences of the oscillator not starting, especially in the hands of the consumer, are expensive, it is advisable to take as many opportunities as possible to ensure that the oscillator is running.